1. Field of The Invention
The present invention relates to a method for estimating the relative speed between a vehicle and an objective obstacle.
2. Description of The Related Art
A collision preventing system for a vehicle is known, for example, from Japanese Patent Publication No. 4700/86, in which the distance between a subject vehicle and an objective obstacle is measured by a distance measuring unit, and a brake device is operated in accordance with the distance between the subject vehicle and the objective obstacle.
In the above collision system for the vehicle, it is necessary to correctly obtain the relative speed between the subject vehicle and the objective obstacle. However, it is inevitable that noise data are incorporated into objective obstacle data obtained in the distance measuring unit.
In the above prior art collision preventing system for the vehicle, the distance is calculated on the basis of the time between the transmission of a pulse signal and the reception of a first reflection from the objective obstacle in response to the pulse signal and the relative speed is estimated from a variation in such calculated distances. This causes four problems:
First when noises, due to the reflection from a road surface and a matter suspended in the air, are received in a condition in which another vehicle, as an objective obstacle, is travelling ahead of the subject vehicle as shown in FIG. 12A, the noises A and B cannot be distinguished from the signals of the ahead-travelling vehicle, as shown in FIG. 12D. This causes a dispersion in relative speed between the subject vehicle and the ahead-travelling vehicle, as shown in FIG. 12E, notwithstanding that there is actually no variation in relative speed between the subject vehicle and the ahead-travelling vehicle. Therefore, it is impossible to correctly determine the behaviour of the ahead-travelling vehicle, thereby causing misoperation.
Second, when another vehicle has cut in between the ahead-travelling vehicle and the subject vehicle, as shown in FIG. 13A, the distance between the subject vehicle and the ahead-travelling vehicle is measured in a section C, while the distance between the subject vehicle and the cutting-in vehicle is measured in a section D from the middle (see FIG. 13D). In such a transition, the relative speed is disordered, as shown in FIG. 13E and, hence, the behavior of the ahead-travelling vehicle and the subject vehicle cannot be correctly confirmed, causing misoperation.
Third, suppose a condition in which a four-wheel vehicle is travelling ahead of a motorcycle which is in turn, travelling ahead of the subject vehicle, as shown in FIG. 14A, where a higher attention should be directed to the four-wheel vehicle, constituting a dangerous situation for collision. In this condition, the presence of the ahead-travelling four-wheel vehicle cannot be detected until the motorcycle runs away from between the subject vehicle and the ahead-travelling four-wheel vehicle, as shown in FIG. 14C. Hence, when the motorcycle runs past the four-wheel vehicle as a result of a speed reduction of the four-wheel vehicle there is a possibility that the judgement of collision of the vehicle, against the ahead-travelling four-wheel vehicle, is retarded.
Fourth, when the subject vehicle is travelling on a curved road, having corner reflectors E.sub.1 to E.sub.8, such reflectors E.sub.1 to E.sub.8 are sequentially detected, as shown in FIG. 15D. Therefore, not only the behavior of the ahead-travelling vehicle cannot be detected, but also there is the possibility of a misoperation due to the dispersion in relative speed, as shown in FIG. 15E.